1. Field of the Invention
The present invention relates to a flow measuring apparatus having a film resistor which serves as a temperature detecting means as well as an electric heater. Such a flow measuring apparatus can be used, for example, for measuring the flow rate of engine intake air.
2. Description of the Related Art
Generally, in an internal combustion engine, the amount of intake air is one of the most important parameters for controlling the fuel injection amount, the ignition timing, or the like. A flow measuring apparatus, i.e., an airflow meter, is provided for measuring the same. One of the more common prior art airflow meters is the vane-type, but this is disadvantageous in scale, response speed characteristics, and the like, and therefore, airflow meters having temperature-dependent resistors have been developed, in which these disadvantages of scale, response speed characteristics, and the like are avoided.
A prior art airflow meter may consist of a film resistor which serves not only as an electric heater, but also as a temperature-detecting means for detecting the temperature of the heated air. The airflow meter may consist of a temperature-dependent resistor for detecting the temperature of non-heated air. Thus, the current flowing through the film resistor is controlled to provide a constant difference in temperature between the film resistor and the temperature dependent resistor, thereby detecting the voltage applied to the film resistor as representing the mass flow rate of air. In this airflow meter, if a temperature-dependent resistor is not provided and the current of the heater resistor is controlled to provide a constant temperature of the film resistor, the voltage applied to the film resistor is detected as representing the volume flow rate of air (see: U.S. Pat. Nos. 4,627,279 and 4,688,425).
In the above-mentioned airflow meter, the film resistor may consist of an insulating substrate such as a ceramic substrate or monocrystalline silicon substrate, a resistance layer of platinum (Pt), gold (Au), etc. on the insulating substrate, and a passivation layer on the resistance pattern. In this airflow meter, the flow rate of air is remarkably reduced around a stagnation region or at an edge on the upstream side around boundary regions. Therefore, suspended particles (mainly, carbon particles) in the air stream easily deposit on the film resistor near the stagnation region and the boundary regions. As a result, the heat capacity is increased and the heat dissipation characteristic thereof is reduced, thus reducing the sensitivity and response speed characteristics of the airflow meter.